Oxygen saturation is a clinically very relevant parameter to assess the condition of a patient. Particularly in the operating room, the oxygen saturation of the blood gives an indication of the patient's condition, its supply with oxygen and other physiological factors.
One possibility to obtain a very precise value of the patient's oxygen saturation is to take a blood sample and analyze it in a blood gas analyzer. Despite the high precision of this method, it is an invasive technique and this means that it cannot performed frequently, i.e. does not allow continuous monitoring. Therefore, significant changes in the oxygen saturation value may be missed. Last not least, it is understood that an invasive technique is not the preferred way to monitor a patient.
It is therefore highly desirable to measure oxygen saturation non-invasively. This can be achieved by a technique called oximetry.
An oximeter usually comprises two or more light sources of different wave length. The light is irradiated on human flesh, and either the intensity of the light transmitted through the flesh, or the intensity of the reflected light is measured. In more general terms, "light" does not only mean electromagnetic waves in the visible spectrum. For example, the most common oximeters use one wavelength in the visible spectrum and another wavelength in the infrared spectrum. Such a oximeter is described for example in "A New Family of Sensors for Pulse Oximetry", S. Kastle, F. Noller et al, February 1997, Hewlett-Packard Journal. For more details of the theory of oxygen saturation measurement, reference is made to former publications on this subject. e.g. U.S. Pat. No. 4,167,331 or EP-A-262778 (the latter patent application contains a quite complete breakdown of the theory).
For obtaining a saturation value, a set of a value quadruple is always necessary which consists of a pair of values for each of the two wavelengths. e.g. red and infrared. (R.sub.1, IR.sub.1) and (R.sub.2, IR.sub.2). Normally, a first pair of values is used as raw curve samples at time 1 and a second pair of values as raw curve samples at time 2. The assumption underlying this course of action is that the samples differ at time 1 and time 2 only with regard to a change in level caused by a change in the arterial blood volume. Normally, the diastolic Pleth value (maximum) is used as the first pair of values and the systolic Pleth value (minimum) is used as the second sample.
Speaking more generally, arbitrary, e.g. composite and/or averaged values for red R and infrared IR can be used, provided that the pairs R and IR belong together signalwise, and provided that the data underlying the pairs of values 1 and 2 only differ with regard to a change in the arterial blood.
A ratio of the two pairs of values can then be calculated as follows: EQU ratio=ln(R.sub.1 /R.sub.2)/ln(IR.sub.1 /IR.sub.2) (1)
The oxygen saturation can then be calculated on the basis of this ratio in the manner known. EQU Sp02=f(ratio) (2)
In the method described hereinbefore, the raw signals obtained on the basis of the intensity of electromagnetic waves or the composite or averaged values obtained from said raw signals are considered in the time domain for determining the oxygen saturation. However, if a disturbance signal exists, it is impossible to separate the useful signal and the disturbance signal in the time domain if they are constantly present. The least-square-x/y method in the time domain is normally always falsified and, if the disturbances are strong disturbances in the order of S/N=1, it is no longer of any use.